1. Field
The disclosed concept pertains generally to electrical systems and, more particularly, to electrical power systems that are subject to arc flashes. The disclosed concept also pertains to arc flash detection apparatus.
2. Background Information
Electric power systems incorporate switches for control and protection purposes. Distribution systems, which form part of the overall electric power system, include main and feeder power buses and circuit breakers mounted in metal cabinets to form switchgear. Interruption of current flow in the buses of the distribution system by a circuit breaker creates an arc as the contacts of the circuit breaker open. These arcs caused by interruption are generally contained and extinguished in the normal course of operation of the circuit breaker.
At times, however, unintended arcing faults can occur within switchgear cabinets, such as between power buses, or between a power bus and a grounded metal component. Such arcing faults can produce high energy gases, which pose a threat to the structure and nearby personnel. This is especially true when maintenance is performed on or about live power circuits. For example, a worker might inadvertently short out the power bus, thereby creating an arcing fault inside the enclosure. The resulting arc blast creates an extreme hazard and could cause injury or even death. This problem is exacerbated by the fact that the enclosure doors are typically open for maintenance.
A common approach to protecting personnel from arcing faults in switchgear has been to design the metal enclosures to withstand the blast from the arcing fault. This has been done at great additional costs due to the heavy gauge metal used and numerous weld joints needed to prevent flying debris. Even with these precautions, the blast from an arcing fault inside the switchgear may not be contained.
Various known methods seek to minimize the severity of the blast from an internal arcing fault. These methods include pressure sensing and light detection, which sense the arcing fault within the switchgear and cause a circuit breaker to trip before significant damage can result. The pressure sensing method is limited by the insensitivity of the pressure sensors. By the time cabinet pressure has risen to detectable levels, the arcing fault has already caused significant damage.
In an electrical system, an internal arcing fault can occur somewhere inside of the switchgear enclosure, frequently, but certainly not limited to the point where the power cables servicing the load are connected.
In an electrical system, such as, for example, a motor control center, an internal arcing fault could occur within the load center panelboard when, for example, servicing line panelboards. A bare live copper bus could inadvertently be shorted. Another example for both low and medium voltage systems would be the shorting of power conductors by rodents, snakes, or other animals or objects.
In the low voltage system, the arcing fault could clear itself, by burning or ejecting the short, but it may take more than one-half cycle to do so, thereby causing significant damage and great risk of injury to workers even in one-half cycle of arcing.
A medium voltage system could behave similar to a low voltage system; however, the medium voltage system would be less likely to be self-extinguishing.
It is known to employ a high-speed shorting switch to eliminate an arcing fault. Known arc elimination devices and systems produce a bolted fault across the power bus (e.g., phase-to-phase, such as two switches for three phases; phase-to-ground, such as three switches for three phases), in order to eliminate the arcing fault and prevent equipment damage and personnel injury due to arc blasts. It is also known to employ various types of crowbar switches for this purpose. The resulting short on the power bus causes an upstream circuit breaker to clear the bolted fault by removing power. See, for example, U.S. Pat. Nos. 7,145,757; 7,035,068; 6,839,209; 6,724,604; 6,693,438; 6,657,150; and 6,633,009. As a result, system power is lost due to the tripping of the upstream circuit breaker. Once the arc is out, and if the short has been burned away or removed, then system power can be restored.
Arc flash light detection systems can employ only the light produced by arcing internal to electrical equipment (see, for example, U.S. Pat. No. 6,229,680), or can sense a combination of light and relatively high current. The addition of current sensing is intended to avoid nuisance operation for normal light sources (e.g., a camera flash; a flashlight). Protective devices, such as air circuit breakers (i.e., circuit breakers that interrupt current in air), produce arc bi-products during normal operation, such as, for example, copper vapor in the arc plasma exhausted from a circuit breaker's arc chute. Since such protective devices also operate during relatively high current conditions, the normal operation of these protective devices with an open arc chamber produces challenges when attempting to protect such devices against the condition of internal arcing, yet also make them immune to the normal arcing such devices produce during relatively high current protection conditions.
Hence, a problem is that known arc flash detection systems cannot differentiate between arcs due to an internal fault versus arcs emanating from an open circuit interrupter.
There is room for improvement in electrical systems.
There is also room for improvement in arc flash detection apparatus.